Masterbatch for manufacturing a drilling fluid

ABSTRACT

An oil-based masterbatch in the form of granules containing carbon nanotubes. Also, a method for preparing an oil-based masterbatch in the form of granules containing carbon nanotubes. Also, the use of an oil-based masterbatch in the form of granules containing carbon nanotubes for manufacturing an aqueous or organic viscoelastic fluid, intended for drilling in underground formations.

The present invention relates to an oil-based masterbatch containingcarbon nanotubes to a process for producing it and to its use for themanufacture of a viscoelastic fluid intended for drilling insubterranean formations.

Drilling fluids, also called drilling muds, are complex fluids used fordrilling oil wells. They are injected, in general continuously, via thedrill pipe string, into the borehole. Their many functions comprise forexample bringing rock debris up to the surface, maintaining ahydrostatic pressure in the borehole sufficient to prevent collapse ofthe rock formation penetrated, or else lubricating and cooling thedrilling head. They are mainly two families of drilling muds: oil-basedmuds (generally inverse emulsions of brine in an oil phase) andwater-based muds.

Maintaining a sufficient hydrostatic pressure, to compensate for thelateral pressure of the rock formation penetrated by the drill well,requires the density of the drilling fluid to progressively incredse asthe drilling advances towards deeper zones. This density increase isobtained by adding weighting agents, that is to say finely ground solidmaterials of high density that are insoluble in the drilling fluid. Thedeeper the drilling well, the larger the amount and/or the higher thedensity of the weighting agent used, and the more effective the systemfor keeping the weighting system in suspension in the drilling fluid hasto be.

This is because any settling of the weighting agent, for example duringa temporary stoppage in mud injection, may have disastrous consequencessuch as a well being blocked or a local reduction in the hydrostaticpressure of the fluid column, resulting in collapse of the well.

The weighting agent is typically kept in suspension by viscosity agents,conventionally chosen from clays, whether or not organophilic, andorganic polymers soluble in the drilling fluid. Now, beyond a certaindrilling depth, when the system for keeping the weighting agent insuspension has to be particularly effective, organic polymers undergodegradation due to the high temperatures prevailing at these depths andbecome partially or completely ineffective.

The problem of thermal degradation of organic polymers cannot be solvedsimply by replacing them with clays. Admittedly, the claysconventionally used as thickening agents (bentonite, montmorillonite,attapulgite and organophilic clays) withstand markedly highertemperatures than organic polymers, but for drilling at great depth theamount of clay necessary to keep large amounts of very dense weightingagent in suspension is considerable. Drilling muds then have anexcessively high solids content, which possesses problems of keepingmuds in circulation due to an excessively high viscosity.

The systems currently used, which are based either on polymers or clays,unfortunately does not enable the weighting agent to be kept insuspension above a temperature of about 250° C.

To alleviate these drawbacks of the conventional thickening agents, theApplicant proposed in application WO 2009/030868 to substitute them,completely or partly, with carbon nanotubes (CNTs) having a particularmean diameter and a particular specific surface area. CNTs exhibitexcellent heat stability and enable large amounts of very denseweighting agent, such as barite or calcite, without thereby giving thedrilling fluid an excessively high viscosity that would in particularimpair its pumpability.

As recommended in the above document, it is desirable for thecomposition of this drilling fluid to be modified over the course of thedrilling process, by progressively increasing the CNT content as thedrilling depth, the drilling temperature and/or the drilling pressureincreases. However, the CNTs are introduced into the drilling fluid inpowder form, a form not very easy for handling by drilling mudmanufacturers. Furthermore, CNT metering is difficult and handling CNTscan lead to dispersing them in the environment, requiring the use ofcomplex systems of hoods and filters, rooms under reduced pressure andother systems for limiting their dispersion.

It would therefore be desirable to have a means for overcoming thisdrawback while at the same time enabling carbon nanotubes to be easilydispersed in the liquid base of the drilling fluid.

The Applicant has been able to develop a masterbatch in solid form,containing a large amount of carbon nanotubes and at least one oil,which meets this requirement.

Document U.S. Pat. No. 4,735,733 does teach, admittedly, oilycompositions with a high CNT content. However, these are greasescontaining 5 to 15% by weight of carbon nanotubes, having a meandiameter of 50 to 200 nm and a specific surface area of less than 190m²/g, which are added to a lubricating oil, such as a mineral oil,having a kinematic viscosity of at least 25 mm²/s. These greases, whichtake the form of a viscoelastic gel at room temperature, are neitherexplicitly intended, nor adapted, to be used as CNT masterbatches forthe manufacture of drilling fluids.

Other compositions containing CNTs combined with an oil are disclosed inthe documents FR 2 893 946, WO 03/050332, WO 2005/060648, WO2005/037966, WO 2006/076728, EP 1 995 274, JP2007-231219 and US2007/293405. In these documents, none of which relates to the field ofdrilling for oil, the compositions containing high CNT contents are notin the form of granules or contain, in addition to the CNTs and oil(silicone or chloroparaffin oil), a resin. These compositions aretherefore not adapted for use as masterbatches for the manufacture ofdrilling fluids either.

A first subject of the present invention is specifically a masterbatchin the form of granules comprising at least one oil, selected from agiven list, and more than 3% by weight of carbon nanotubes having a meandiameter of between 5 and 30 nm and a specific surface area greater than200 m²/g, preferably between 200 m²/g and 350 m²/g. The oils that can beused in this embodiment of the invention are chosen from: vegetable oilshaving a high triglyceride content, consisting of fatty acid andglycerol esters; synthetic oils of formula R5COOR6 in which R5represents an aryl group or the residue of a linear or branched, higherfatty acid containing 7 to 30 carbon atoms, and R6 represents a branchedor unbranched hydrocarbon chain, optionally hydroxylated, containing 3to 30 carbon atoms; synthetic ethers; linear or branched, saturated orunsaturated, C₆-C₂₆ fatty alcohols; oils of mineral origin; cyclichydrocarbons such as (alkyl)cycloalkanes and (alkyl)cycloalkenes, thealkyl chain of which is linear or branched, saturated or unsaturated,having 1 to 30 carbon atoms; aromatic hydrocarbons; fluorinated oilssuch as C₈-C₂₄ perfluoroalkanes; fluorosilicone oils; and mixturesthereof

A second subject of the present invention is a masterbatch in the formof granules, consisting of at least one oil and more than 3% by weightof carbon nanotubes having a mean diameter of between 5 and 30 nm and aspecific surface area greater than 200m²/g, preferably between 200 m²/gand 350 m²/g.

These masterbatches make it possible in particular to resolve theabovementioned drawbacks. The CNTs can be precisely metered by the mudmanufacturer and easily handled without involving a complexantidispersion system.

The carbon nanotubes used in the present invention are known. They haveparticular crystalline structures, which are tubular and hollow, with amean length/mean diameter ratio of greater than 1 and made up of one orseveral sheets or walls of graphite, wound coaxially around thelongitudinal axis of the nanotubes. Single-wall nanotubes or SWNTs arethus distinguished from multi-wall nanotubes or MWNTs. According to theinvention, multi-wall CNTs are advantageously used which may for examplecomprise 3 to 15 sheets and more preferably 5 to 10 sheets.

As indicated above, the CNTs used in the present invention have a meandiameter, measured by transmission electron microscopy, of between 5 and30 nm, preferably ranging from 8 to 15 nm. Their mean length isadvantageously between 0.1 and 10 μm. The mean length/mean diameterratio is advantageously greater than 10 and usually greater than 100.

The specific surface area of the CNTs used in the present invention,determined by the nitrogen adsorption BET method, is greater than 200m²/g and preferably between 200 m²/g and 350 m²/g, for example between200 m²/g and 250 m²/g. Their untamped bulk density is preferably between0.03 and 0.5 g/cm³ and in particular between 0.05 and 0.2 g/cm³. Thisbulk density is the ratio of a given mass of carbon nanotubes divided bythe volume of this same mass, measured after three successive inversionsof a specimen containing said nanotubes.

The carbon nanotubes of small mean diameter and high specific surfacearea used in the present invention may especially be prepared accordingto the synthesis processes described in International application WO2006/082325.

Raw carbon nanotubes, i.e. chemically unmodified CNTs, having the abovetechnical characteristics, are also commercially available from thecompany ARKEMA under the brand name Graphistrength® C100.

According to the invention, the nanotubes may be purified and/oroxidised and/or milled before being incorporated into the masterbatch ofthe present invention.

The CNTs may be milled, either hot or cold, in apparatus such as a boremill, a hammer mill, a grinding wheel mill, a knife mill, a gas jet orany other milling system capable of reducing the size of the entangledCNT network. Preferably this milling step is carried out using a gas-jetmilling technique and preferably in an air-jet mill.

The raw or milled CNTs may he purified by washing them in a sulphuricacid solution, so as to strip them of any residual mineral and metallicimpurities coming from their production process. The CNT/sulphuric acidweight ratio used for this washing may be between 1/2 and 1/3. Thepurification operation may also be carried out at a temperature rangingfrom 90 to 120° C., for example for a time of 5 to 10 hours. Thisoperation may advantageously be followed by steps of washing thepurified CNTs in water and drying them.

The raw, milled and/or purified CNTs may be oxidised by bringing thenanotubes into contact with a sodium hypochlorite solution, for examplein a CNT/sodium hypochlorite weight ratio ranging from 1/0.1 to 1/1,preferably at room temperature. This oxidation operation isadvantageously followed by steps of filtering and/or centrifuging,washing and drying the oxidised CNTs.

The CNTs used in the present invention may be chemically modified byintroducing functional groups via covalent bonds. These functionalgroups, such as sulphate, sulphonate, carboxyl, benzenesulphonate andoptionally quaternized amine groups, or else groups obtained by thepolymerization of monomers on the surface of the CNTs, generally improvethe dispersibility of the nanotubes in water or organic solvents.

Furthermore, the CNTs may undergo a heat treatment of at least 900° C.and, better still, at least 1000° C., for the purpose of removing themetallic traces of their synthesis catalyst that may possibly bepresent.

In the present invention, unmodified CNTs are preferably used.

The masterbatch according to the invention contains more than 3% byweight, but generally from 10% to less than 60% by weight, relative tothe total weight of the masterbatch, of carbon nanotubes, preferablyfrom 10 to 55% by weight and more preferably still from 15 to 50% byweight, of carbon nanotubes.

The oil included in the masterbatch according to the invention isdefined as a medium which is liquid at room temperature (25° C.) and atatmospheric pressure and is immiscible in water (i.e. forming 2 phasesvisible to the naked eye at room temperature and atmospheric pressure).This liquid medium has in particular a water solubility, measuredaccording to the OECD TG 105 method, of 1 mg/l or less. This liquidmedium may be relatively viscous. In particular, it has a dynamicviscosity at room temperature of between 0.1 cP and 500 cP andpreferably between 0.3 and 300 cP.

According to the invention, one or more mutually miscible oils may beused. These oils may be polar oils or, better still, apolar oils.

Examples of oils suitable for use in the present invention comprise:

-   -   vegetable oils having a high (for example at least 50 wt %)        content of triglycerides, consisting of esters of fatty acids        and of glycerol, the fatty acids of which can have varied chain        lengths, it being possible for these chains to be linear or        branched and saturated or unsaturated. These oils are especially        the following: wheat germ, maize, sunflower, linseed, shea,        castor, sweet almond, macadamia, apricot, soybean, cottonseed,        alfalfa, poppy seed, pumpkin seed, sesame, cucumber, avocado,        hazelnut, grape seed, blackcurrant seed, evening primrose,        millet, barley, quinoa, olive, rye, safflower, Kendal nut,        passionflower or musk rose oil; or else triglycerides of        caprylic/capric acids;    -   synthetic oils of formula R₅COOR₆ in which R₅ represents an aryl        group or the residue of a higher, linear or branched, fatty acid        having 7 to 30 carbon atoms and R₆ represents a branched or        unbranched hydrocarbon chain, possibly hydroxylated, containing        3 to 30 carbon atoms, such as for example the oil PurCellin®        (the octanoate of cetostearyl alcohol), isononyl isononanoate,        benzoates of C₁₂ to C₁₅ alcohols, isostearyl benzoate, isopropyl        myristate, octanoates, decanoates or ricinoleates of alcohols or        polyalcohols;    -   synthetic ethers, such as petroleum ether;    -   linear or branched, saturated or unsaturated, C₆ to C₂₆ fatty        alcohols, such as oleic alcohol or octyldodecanol;    -   optionally, silicone oils such as: polydimethylsiloxanes that        are liquid at room temperature; polydimethylsiloxanes having        pendant alkyl or alkoxy groups and/or alkyl or alkoxy groups at        the end of a silicone chain, which groups have from 2 to 24        carbon atoms; phenylated silicones, such as phenyl        trimethicones, phenyl dimethicones,        phenyl(trimethylsiloxy)diphenylsiloxanes, diphenyl dimethicones        and diphenylmethyldiphenyltrisiloxanes;    -   mineral oils such as linear or branched hydrocarbons, such as        paraffin oils and derivatives thereof, which are optionally        refined, vaseline, polydecenes, hydrogenated polyisobutene, such        as Parleam®, squalane;    -   cyclic hydrocarbons such as (alkyl)cycloalkanes and        (alkyl)cycloalkenes, the alkyl chain of which is linear or        branched and saturated or unsaturated, having from 1 to 30        carbon atoms, such as cyclohexane, dioctylcyclohexane,        2,4-dimethyl-3-cyclohexene and dipentene;    -   aromatic hydrocarbons such as benzene, toluene, p-cymene,        naphthalene and anthracene;    -   fluorinated oils such as C₈ to C₂₄ perfluoroalkanes;    -   fluorosilicone oils;

and mixtures thereof.

It is preferred to use a mineral oil, such as a paraffin oil, such asthose sold by Shell under the brand name Sarapar® or that sold by TOTALunder the brand name EDC® 99-DW or EDC® 95-11. This oil has a viscosityof 3.5 cPs. As a variant, it is possible to use a refined paraffin oil,having a content of polycyclic aromatic compounds of less than 3% byweight (determined by DMSO extraction according to the IP 346 method)and a glass transition temperature below −45° C., for example −58° C.±3°C. (as determined according to the ASTM E 1356 standard). Refined oilsof this type are in particular:

-   -   MES oils, produced by the solvent extraction of heavy petroleum        distillates or by treatment of these distillates with hydrogen        in the presence of a catalyst (hydrotreatment);    -   TDAEs, which are treated distillate aromatic extracts.

Examples of such oils are in particular sold by Shell under the brandname Catenex® SNR, by Exxon Mobil under the brand name Flexon® 683, byTotal under the brand name Plaxolene® MS or by H&R European under thebrand name Vivatec® 500.

Although the above oils are preferred, it is possible, if appropriate,to use more viscous oils, usually called greases. The size of the CNTsand their specific surface area make it possible to manufacturegrease-based masterbatches suitable for oil drilling, unlike the CNTsrecommended in the prior art.

Examples of greases are in particular:

-   -   industrial greases, such as the complex calcium sulphonate        greases, available in particular in the CERAN range or under the        AXA GR1 reference from the company TOTAL;    -   speciality greases, such as semi-synthetic greases,        bentone-based greases, optionally containing fluorinated        compounds, lithium/calcium soap greases and copper-containing        greases, particularly those available from the company TOTAL        under the brand names MARSON SY 00 and SY 2, SPECIS CU, CALORIS        23 and MS23, STATERMIC XHT and NR, TIFORA PG, BIOMULTIS SEP 2,        MULTIS COMPLEX HV2; and    -   mixtures thereof.

The oils may represent 40 to 96.9% by weight, preferably 45 to 90% byweight and better still 50 to 70% by weight relative to the weight ofthe masterbatch.

In a first embodiment of the invention, apart from CNTs and oil, themasterbatch according to the invention may contain additives such asviscosifiers, salts, especially triethanolamine salts, present in asmall amount, that is to say representing in total, for example, 0 to10% by weight relative to the weight of the masterbatch. Alternatively,in a second embodiment of the invention, the masterbatch consists onlyof CNTs and the oil or oils.

The viscosifiers may be chosen from thickening homopolymers andcopolymers, it being possible for the copolymers to be random or blockcopolymers, thickening oligomers, surfactants and mixtures thereof.

Examples of thickening polymers are those conventionally used indrilling fluids, and examples that may be mentioned include: guar gum,hydroxypropylguar, carboxymethylguar, hydroxypropylcellulose,hydroxyethylcellulose, sodium carboxymethylcellulose, xanthan gum,starch, polyacrylates, poly(diallyldimethylammonium chloride) andmixtures thereof.

The surfactants may be chosen from all non-ionic, anionic, cationic,amphoteric or zwitterionic surfactants capable of forming micelles(whether spherical, cylindrical, tubular or other forms) above a certainconcentration, called the critical micelle concentration. Examples ofsuch surfactants are given in the application EP 1 634 938.

The invention also relates to a process for producing the masterbatchaccording to the first or the second embodiment of the invention.

This process comprises the steps consisting in:

(a) introducing and then kneading the nanotubes and said at least oneoil in a compounding device in order to obtain a composite;

(b) extruding and then cooling said composite so as to obtain amasterbatch in solid form; and

(c) forming said masterbatch in order to obtain granules.

It is well understood that the above process may include other,preliminary, intermediate or subsequent, steps in addition to thosementioned above. Thus, it may especially include a preliminary step ofmixing the nanotubes with the oil, either in the compounding apparatusor using a separate mixer.

Compounding devices are well known to those skilled in the art. They areconventionally used in the plastics industry for melt-blendingthermoplastic polymers and additives in order to produce composites.These devices generally include feed means, especially at least onehopper for pulverulent materials (here, the CNTs) and/or at least oneinjection pump for liquid materials (here, the oil); high-shear mixingor kneading means, for example a co-rotating or counter-rotatingtwin-screw extruder (for example a DSM microextruder) or a co-kneader,usually comprising a feed screw placed in a heated barrel (or tube); anoutput head, which gives the extrudate its shape; and means for coolingthe extrudate, either by air cooling or by circulation of water. Theextrudate is generally in the form of strands continuously exiting thedevice and able, after being cooled, to be cut or formed into granules.However, other forms may be obtained by fitting a die of desired shapeon the output head.

According to the inventors, the solid form would be due to adsorption ofthe oil by the CNTs.

Examples of co-kneaders that can be used according to the invention areBUSS® MDK 46 co-kneaders and those of the BUSS® MKS or MX series, soldby the company BUSS AG, which all consist of a screw shaft provided withflights, placed in a heated barrel optionally made up of several parts,and the internal wall of which is provided with kneading teeth designedto cooperate with the flights so as to shear the kneaded material. Theshaft is rotated, and given an oscillatory movement in the axialdirection, by a motor. These co-kneaders may be equipped with agranulation system, for example fitted at the exit orifice of saidco-kneaders, which may consist of an extrusion screw.

The co-kneaders that can be used according to the invention preferablyhave an L/D screw ratio of 7 to 22, for example 10 to 20, whereasco-rotating extruders advantageously have an L/D ratio from 15 to 56,for example 20 to 50.

The compounding step was generally carried out at a temperature close(for example within ±5° C.) to room temperature (25° C.).

The masterbatch according to the invention, obtained for example usingthe process described above, may be diluted in a liquid base intendedfor manufacturing a drilling fluid.

Another subject of the present invention is therefore the use of themasterbatch described above to manufacture a viscoelastic fluid fordrilling in subterranean rock formations, which comprises mixing saidmasterbatch with an aqueous and/or organic liquid base and optionally atleast one weighting agent in particulate form.

Yet another subject of the present invention is a process formanufacturing a viscoelastic fluid for drilling in subterranean rockformations, which comprises mixing the masterbatch described above withan aqueous and/or organic liquid base and optionally at least oneweighting agent in particulate form.

The masterbatch may be introduced into the liquid base optionallycontaining the weighting agent in suspension. More particularly,according to one embodiment of the invention, the masterbatch isincorporated directly into the final formulation of the viscoelasticdrilling fluid. However, it is preferred to add the weighting agent tothe liquid base after the masterbatch has been introduced into saidbase.

Introducing CNTs into a liquid base for a drilling fluid (or mud) iseasily performed using the masterbatch of the invention, unlike directlyintroducing CNTs into the drilling fluid, and especially on the actualdrilling site.

The liquid base may in principle be any base conventionally used indrilling fluids. For example, it may be an aqueous base, advantageouslyused for economic and environmental reasons. These aqueous basescontain, as is known, water-soluble salts intended mainly to increasethe density of the base. The preferred salts comprise sodium, potassium,calcium, zinc and caesium halides and formates, and combinationsthereof. Particularly preferred salts that may be mentioned are calciumchloride, calcium bromide, potassium formate, caesium/potassium formateand combinations thereof. These aqueous bases may also contain smallfractions of water-miscible and/or water-immiscible organic solvents.The solvents may possibly be dispersed in the form of droplets in theaqueous base in order to form a dispersion/suspension or an oil-in-water(O/W) emulsion.

In the aforementioned cases, introducing the masterbatch into theaqueous base results in the formation of a drilling fluid in the form ofa suspension, dispersion or emulsion, especially an oil-in-water (O/W)emulsion.

As a variant, it may be beneficial, or even necessary, to limit thewater content of the drilling fluids, for example when the well ispassing through rock formations containing a large fraction ofwater-soluble or water-dispersible components liable to be carried awayby the fluid. The liquid base may then be either an oil, or asuspension, dispersion or water-in-oil (W/O) emulsion preferablycontaining at most 50% by weight, and in particular at most 20% byweight, of water. Introducing the masterbatch into these bases thereforeresults in a drilling fluid in the form of an oil, a dispersion or anoil-in-water-in-oil (O/W/O) multiple emulsion, respectively.

When the liquid base is in the form of a dispersion or a W/O or O/Wemulsion, manufacture of the viscoelastic fluid then also involvesintroducing at least one surfactant (a W/O or O/W emulsifier,respectively) into the liquid base, or introducing a dispersant capableof stabilising the emulsion or dispersion respectively, which may bechosen from nonionic or anionic surfactants and may represent forexample from 1 to 5% by weight relative to the total weight of thedrilling fluid.

The oil constituting the liquid base or else the oil forming thecontinuous phase of the dispersion or water-in-oil emulsion, or thediscontinuous phase of the dispersion or oil-in-water emulsion, ispreferably a mineral oil, a fluorinated oil, a diesel oil or a syntheticoil, preferably a mineral oil and especially a hydrocarbon mixture or asynthetic oil. In general, apolar oils are preferred to polar oils.Advantageously, the oil of the liquid base is identical to that of themasterbatch. An oil conventionally used is for example a paraffin oilsuch as the commercial product EDC® 99-DW from the company TOTAL.

The liquid base described above also optionally contains at least oneweighting agent. In principle, any particulate solid having a densitygreater than that of the liquid base, preferably a density of at least 2g/cm³, and, for very deep drilling, preferably a density greater than 3g/cm³, or even greater than 4 g/cm³, may be used as weighting agent.These weighting agents are known and chosen for example from barite(BaSO₄), calcite (CaCO₃), dolomite (CaCO₃.MgCO₃), hematite (Fe₂O₃),magnetite (Fe₃O₄), ilmenite (FeTiO₃) and siderite (FeCO₃). The weightingagent used is particularly preferably barite.

The amount of weighting agent essentially depends on the desireddrilling fluid density. This density, and therefore the amount ofweighting agent used, increases in general progressively with the depthof the drilling well. The drilling fluids of the present invention arepreferably intended for very deep drilling and consequently have arelatively high density, preferably an overall density of at least 1.5,preferably greater than 2.5. The upper limit of the content of weightingagent is essentially determined by the viscosity problems that too higha solids content entails. Generally, the weighting agent is used in thedrilling fluid manufactured according to the present invention with aconcentration between 10 and 70% by weight, for example between 20 and50% by weight. The percentage concentration of weighting agent can varygreatly depending on the desired density.

The amount of masterbatch introduced into the drilling fluids of thepresent invention depends, inter alia, on the amount and the density ofthe weighting agent used, on the drilling depth, on the nature of thebase liquid and on whether or not other thickening agents are present inthe drilling fluid.

This amount of masterbatch is preferably such that the carbon nanotubesrepresent 0.1 to 6% by weight, for example 0.1 to 3% by weight, relativeto the total weight of the drilling fluid.

It is also preferable according to the invention for the masterbatch andthe liquid base to be subjected to a mechanical treatment, preferablybefore being mixed with the weighting agent. This treatment may be ofany type, provided that it makes it possible for the CNTs to beuniformly dispersed in the liquid base. According to the invention, thistreatment preferably comprises an ultrasonic treatment or a shearing ofthe masterbatch dispersion using a rotor-stator system or using a bladekneader.

Such a rotor-stator system generally comprises a rotor driven by a motorand provided with fluid guiding systems perpendicular to the rotor axis,such as paddles or blades placed approximately radially, or a flat discprovided with peripheral teeth, said rotor being optionally providedwith a ring gear, and a stator arranged concentrically with respect tothe rotor, and at a short distance to the outside of the latter, saidstator being equipped, over at least a portion of its circumference,with openings provided for example in a grid or defining between themone or more rows of teeth, which are suitable for passage of the fluiddrawn into the rotor and ejected by the guiding systems towards saidopenings. One or more of the aforementioned teeth may be provided withsharp edges. The fluid is thus subjected to a high shear, both in thegap between the rotor and the stator and through the openings providedin the stator.

One example of a rotor-stator system is in particular that sold by thecompany SILVERSON under the brand name Silverson® L4RT. Another type ofrotor-stator system is sold by the company IKA-WERKE under the brandname Ultra-Turrax®. Yet other rotor-stator systems consist of colloidmills, turbine deflocculators and high-shear mixers of the rotor-statortype, such as the machines sold by the company IKA-WERKE or by thecompany ADMIX.

According to the invention, the speed of the rotor is preferably set atat least 1000 rpm and preferably at least 3000 rpm or even at least 5000rpm, for example for to 15 minutes. Furthermore, the width of the gapbetween the rotor and the stator is preferably less than 1 mm,preferably less than 200 μm, more preferably less than 100 μm and betterstill less than 50 μm or even less than 40 μm. Moreover, therotor-stator system used according to the invention advantageouslyapplies a shear rate ranging from 10³ to 10⁹ s⁻¹.

In one particular embodiment of the present invention, the CNTsconstitute the sole thickening agent, that is to say the drilling fluidis essentially free of other known thickening agents, such as organicpolymers, fatty acids, clays or thickening systems based on surfactantsor on electrolytes, such as those described in EP 1 634 938. The CNTconcentration in the drilling fluid is then relatively high, preferablybetween 1 and 6% by weight, particularly between 1.5 and 3% by weight,relative to the weight of the viscoelastic drilling fluid. This isbecause experience has shown that when other thickening agents areabsent, the yield point of the drilling fluids increases spectacularlyabove a minimum value of the order of 1% by weight of CNT.

The carbon nanotubes are also useful for enhancing the effect ofconventional thickening systems, for example polymer-based thickeningsystems. In another embodiment of the present invention, the drillingfluids obtained according to the present invention thus also contain oneor more organic polymers soluble in the aqueous phase and/or in the oilyphase of the liquid base. The CNT concentration is then preferablybetween 0.1 and 1% by weight relative to the weight of the viscoelasticdrilling fluid.

The drilling fluid obtained from the masterbatch according to theinvention is intended to be used in a process for drilling subterraneanrock formations.

Thanks to the excellent heat resistance of the CNTs used according tothe invention, they are particularly appropriate for very deep drilling,that is to say under high-temperature (generally 200° C. or higher,particularly 250° C. or higher) and high-pressure conditions (generallyabove 10 000 psi for an HTHP well, and even above 30 000 psi for anextreme HTHP well).

During drilling, it is possible for one or more thickening agentspresent in the fluid, chosen for example from clays (bentonite,montmorillonite, attapulgite, organophilic clays) or organic polymers,to be progressively replaced with carbon nanotubes as the drillingdepth, the drilling temperature and/or the drilling pressure increases.It may in fact be worthwhile, mainly for drilling fluid production costreasons, to use known and inexpensive thickening agents, such as organicpolymers and/or thickening clays, at the start of drilling and tointroduce the CNTs only beyond a certain depth when the thermaldegradation of the organic polymers or the excessive solids contentprovided by clays starts to pose the problems described in theintroduction.

The invention will be better understood in the light of the followingexamples, which are given purely by way of illustration and are notintended to limit the scope of the invention as defined by the appendedclaims.

EXAMPLES Example 1 Production of a Masterbatch

Carbon nanotubes (Graphistrength® C.100 from ARKEMA) were introducedinto the feed well of zone 1 of a BUSS® MDK 46 (11 L/D) co-kneader. Anequivalent weight of mineral oil (EDC® 99 DW from TOTAL), i.e. havingthe same weight as that of the CNTs, was introduced into the injectionpump of the first zone of the machine, before the first restrictionring. The kneading was carried out at room temperature. Solids strandswere obtained at the exit of the co-kneader, which were cut, withoutusing water jets, to obtain a masterbatch in the form of solid granulescontaining 50% by weight of CNT and 50% by weight of oil.

Example 2 Production of a Masterbatch

Carbon nanotubes (Graphistrength® C.100 from ARKEMA) were introducedinto the feed well of zone 1 of a BUSS® MDK 46 (11 L/D) co-kneader. Anequivalent weight of mineral oil (EDC® 99 DW from TOTAL), i.e. havingthe same weight as that of the CNTs, was introduced into the injectionpump of the first zone of the machine, before the first restrictionring. The kneading was carried out at room temperature. Solids strandswere obtained at the exit of the co-kneader, which were cut, withoutusing water jets, to obtain a masterbatch in the form of solid granulescontaining 20% by weight of CNT and 80% by weight of oil.

Example 3 Manufacture of a Drilling Fluid

The masterbatch of Example 2 was impregnated with the same mineral oilas that used for its preparation, in an amount of 1 part by weight ofmasterbatch per 2 parts by weight of oil, for at least 8 hours. Thesuspension was then rediluted in the same oil, so as to achieve a CNTcontent of 1% by weight. This suspension was then subjected to amechanical treatment in a rotor-stator system (a Silverson® L4RT machinefrom SILVERSON) for 10 minutes in order to obtain a stable CNTdispersion, that is to say one showing no visible sedimentation after 24hours. A weighting agent such as barite was then introduced into thisdispersion in order to obtain a drilling fluid.

1. Masterbatch in the form of granules comprising at least one oil andmore than 3% by weight of carbon nanotubes having a mean diameter ofbetween 5 and 30 nm and a specific surface area greater than 200 m²/g,said oil being chosen from: vegetable oils having a high content oftriglycerides, consisting of esters of fatty acids and of glycerol;synthetic oils of formula R₅COOR₆ in which R₅ represents an aryl groupor the residue of a higher, linear or branched, fatty acid having 7 to30 carbon atoms and R₆ represents a branched or unbranched hydrocarbonchain, possibly hydroxylated, containing 3 to 30 carbon atoms; syntheticethers; linear or branched, saturated or unsaturated, C₆ to C₂₆ fattyalcohols; mineral oils; cyclic hydrocarbons; aromatic hydrocarbons;fluorinated oils; fluorosilicone oils; and mixtures thereof. 2.Masterbatch in the form of granules consisting of at least one oil andof more than 3% by weight of carbon nanotubes having a mean diameter ofbetween 5 and 30 nm and a specific surface area greater than 200 m²/g.3. Masterbatch according to claim 2, wherein the at least one oil ischosen from oils having a dynamic viscosity at room temperature ofbetween 0.1 cP and 500 cP.
 4. Masterbatch according to claim 1, whereinthe masterbatch contains from 10 to less than 60% by weight of carbonnanotubes.
 5. Masterbatch according to claim 1, wherein the masterbatchcontains 10 to 55% by weight of carbon nanotubes.
 6. Process forproducing a masterbatch according to claim 1, wherein the processcomprises the steps of: (a) introducing and then kneading the nanotubesand said at least one oil in a compounding device in order to obtain acomposite; (b) extruding and then cooling said composite so as to obtaina masterbatch in solid form; and (c) forming said masterbatch in orderto obtain granules.
 7. Process for the manufacture of a viscoelasticfluid for drilling subterranean rock formations, which comprises mixinga masterbatch according to claim 1 with an aqueous and/or organic liquidbase and optionally at least one weighting agent in particulate form. 8.Process for manufacturing a viscoelastic fluid for drilling insubterranean rock formations, which comprises mixing the masterbatchaccording to claim 2, with an aqueous and/or organic liquid base andoptionally at least one weighting agent in particulate form.
 9. Processaccording to claim 8, wherein said weighting agent has a density of atleast 2 g/cm³.
 10. Process according to claim 8, wherein the weightingagent is barite.
 11. Process according to claim 8, wherein theviscoelastic fluid contains 0.1 to 6% by weight of carbon nanotubes. 12.Process according to claim 8, wherein the masterbatch and the liquidbase are subjected to a mechanical treatment using ultrasound or arotor-stator system or a blade mixer.
 13. Process according to claim 8,in which the masterbatch is incorporated directly into the finalformulation of the viscoelastic drilling fluid.
 14. Masterbatchaccording to claim 2, wherein the masterbatch contains from 10 to lessthan 60% by weight of carbon nanotubes.
 15. Process according to claim7, wherein said weighting agent has a density of at least 2 g/cm³. 16.Process according to claim 7, wherein the weighting agent is barite. 17.Process according to claim 7, wherein the viscoelastic fluid contains0.1 to 6% by weight of carbon nanotubes.
 18. Process according to claim7, wherein the masterbatch and the liquid base are subjected to amechanical treatment using ultrasound or a rotor-stator system or ablade mixer.
 19. Process according to claim 7, in which the masterbatchis incorporated directly into the final formulation of the viscoelasticdrilling fluid.
 20. Process for producing a masterbatch according toclaim 2, wherein the process comprises the steps of: (a) introducing andthen kneading the nanotubes and said at least one oil in a compoundingdevice in order to obtain a composite; (b) extruding and then coolingsaid composite so as to obtain a masterbatch in solid form; and (c)forming said masterbatch in order to obtain granules.